In recent years, the requirement for data transfer capacity and reliable networks have increased. Standards, such as ITU-T G984, have been developed in order to increase the speed and accordingly the capacity of optical network systems.
Reliability of communication networks is an increasingly important parameter, and accordingly continuous operation of optical networks in case of breakage of fibers or malfunctioning nodes or devices is desired. The ITU-T G.984 standard specifying GPON includes four different protection switching possibilities in ITU-T G.984.1 (March 2003). These are:                Fiber duplex system (type-A scheme): Only the trunk fiber is duplex. In case of a feeder fiber break, the spare fiber can be switched in manually. Since the switching should be automated in larger networks, fiber switches are necessary which are costly and render this scheme uneconomical.        OLT-only duplex system (type-B scheme): Trunk fiber and optical line termination (OLT) line terminal (LT) are duplex. One OLT-LT is in operation, the other is in hot-standby, kicking in if the OLT-LT fails or if the trunk breaks. Optical Network Units (ONU)/Optical Network Terminals (ONT) and drop fibers are simplex. Since only the components that are shared by the users are duplex, the scheme shows a good tradeoff between costs and fault tolerance.        Full duplex system (type-C scheme): Fully failure tolerant system, since all components are duplex. The whole distribution fiber network has to be doubled, leading to very high costs for this solution.        Partial duplex system (type-D scheme): With a mix of type-B and type-C protection, simplex and duplex users can be mixed on the PON. The scheme proposed in the standard is unworkable due to the fiber-cross in the splitter.        
So far, the interest in protection switching in the GPON community has been limited since typical deployment scenarios support a maximum of 64 split on 20 km reach, i.e. the protection benefits are small compared to the costs of the schemes. However, with the development of reach-extended systems with higher splits (128 to 256) protection will become an essential part of the PON system, since a trunk fiber cut or OLT failure will cause service outage for a high number of users.
Considering a type-B scheme, switch-over based on the standard procedure as proposed in the standard will take several minutes to occur since all ONUs/ONTs move to initial state where a full initialization including activation and ranging is necessary. Thus such schemes cannot recover quickly and connection or session continuity cannot be accomplished.